This invention relates generally to optical signal processing systems and more particularly to beamforming controls for phased array antennas in radar systems.
Phased array antenna systems employ a plurality of individual antenna elements or subarrays of antenna elements that are separately excited to cumulatively produce a transmitted electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase, respectively, so that an equiphase beamfront, or the cumulative wavefront of electromagnetic energy radiating from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna activating signals determines the direction in which the cumulative beam from; all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, or the adjustment of the relative phase of the actuating signals for the individual antenna elements (or subarrays of antennas) can be accomplished by electronically shifting the phases of the actuating signals or by introducing a time delay in the different actuating signals that sequentially excite the antenna elements in order to generate the desired direction of beam transmission from the antenna.
Optical control systems are advantageously used to create selected time delays in actuating signals for phased array antenna systems. Such optically-generated time delays are not frequency dependent and thus can be readily applied to broadband phased array antenna systems. For example, optical signals can be processed to establish the selected time delays between individual signals to cause the desired sequential actuation of the transmitting antenna elements, and the optical signals can then be converted to electrical signals, such as by a high speed photodetector array.
Several architectures for optical time delay units have been proposed. For example, an optical beam forming system for a phased array antenna is disclosed in U.S. Pat. No. 5,117,239 of N. Riza entitled "Reversible Time Delay Beamforming Optical Architecture for Phased Array Antennas," which is assigned to the assignee of the present invention and incorporated herein by reference. These architectures generally depend on the use of linearly polarized light so that light beams of a predetermined polarization are directed through particular paths in the architecture to generate the differential time delay between a delayed and an undelayed signal. Thus, controlling the polarization of a light beam entering the architecture determines the path that the light beam follows, and the path determines the delay imparted to the light beam.
The optical control system disclosed in the above referenced patent includes a transmit/receive phased array beamformer for generating true-time-delays using optical free-space delay lines and two dimensional liquid crystal spatial light modulators for implementing the optical switching. Unlike conventional optical switching techniques, the liquid crystal-based optical switching elements can provide low insertion loss and low crosstalk level switching with relatively easily fabricated and low cost liquid crystals.
In these polarization based systems using arrays of nematic liquid crystals (NLCs) and polarizing beam splitters to generate the time delay used in controlling the antenna, several factors can cause system performance to be degraded. For example, cube beam splitters of the type typically have an extinction ratio of about 1000:1. That is, the intensity of the light passing from a given output port of the cube polarizing beam splitter (e.g., the p-polarized light port)is only one-thousand times the intensity of light of the opposite linear polarization (s-polarized light in this example) also passing from that output port. A device having extinction ratios of this magnitude provides an electrical signal to noise ratio of about 60 dB. Such a signal to noise ratio provides good optical signal processing system performance. In some applications, however, even better signal to noise ratios are advantageous, such as to enhance the detectability of long range targets or targets having a small radar cross section.
It is accordingly an object of this invention to provide a high performance optical signal processing system with optical time delay units having a signal to noise ratio better than that exhibited by systems using cube polarizing beam splitters.
It is a further object of this invention to provide a phased army radar system exhibiting improved detection of long range or small radar cross section targets.